The invention relates to a process for the removal of glycoalkaloids, in particular from process streams such as those encountered during isolation of proteins from potatoes.
The potato belongs to the Solanaceae, or nightshade, family whose other members include tomatoes, eggplants, peppers and tomatillos. The proteins that can be found in potatoes have great nutritional value. The nutritional qualities, i.e. protein efficiency ratio and biological value, of these proteins have been shown to be greater than those of casein and comparable to those of whole egg. Potato protein is rich in lysine and theoretically an excellent supplement for lysine-poor proteins such as those of cereals.
Native potato proteins can tentatively be divided into three classes (i) the patatin family, highly homologous acidic 43 kDa glycoproteins (40-50 wt. % of the potato proteins), (ii) basic 5-25 kDa protease inhibitors (30-40 wt. % of the potato proteins) and (iii) other proteins mostly high molecular weight proteins (10-20 wt. % of the potato proteins) (Pots et al., J. Sci. Food. Agric. 1999, 79, 1557-1564). Patatin is a family of glycoproteins that have lipid acyl hydrolase and transferase activities and can account for up to 40% of the total soluble protein fraction in potato tubers.
Potato proteins may be isolated from potato fruit juice. In the professional vocabulary, the undiluted juice from the potato tuber is called potato fruit juice (PFJ), whereas the diluted juice is designated potato fruit water. Both have a high content of organic materials which give rise to high oxygen demand in waste water from the potato starch plants. The potato fruit water also contains phosphorous- and nitrogen-compounds which fertilize the recipients. Some potato starch manufacturers employ evaporation or reverse osmosis to concentrate potato fruit water for use as feed supplement. Reverse osmosis, which is not as energy demanding as evaporation, does however demand that the potato fruit water is pre-treated and filtered to clarity to avoid clogging of the membranes which hold inorganic salts and low molecular weight organic components back in the concentrate.
Fresh potato juice is a complex mixture of soluble and insoluble material comprising proteins, starch, minerals, toxic glycoalkaloids, fibers and monomeric and polymeric reactive phenols. Due to oxidation of natural phenolic compounds potato juice may turn brown or black. Chemically, the phenolic compounds are oxidized into quinones, which rapidly combine into a dark polymer residue. During the oxidation process, the proteins may undergo rapid reaction and partial crosslinking. This crosslinking dramatically reduces the solubility of the proteins, potentially resulting in sedimentation. Thus, from a technological point of view, the complexity and instability of the potato juice makes the separation and isolation of minimally denatured or modified potato proteins much more complicated and economically demanding than the isolation of proteins from other types of protein solution, such as ewe or cow milk.
Another complication of purification of potato proteins is formed by the presence of glycoalkaloids, which must be removed before the potato proteins may be used in human nutrition and human applications. Glycoalkaloids are well-known anti-nutritional factors. The glycosylated forms of glycoalkaloids, such as α-solanine and α-chaconine, show the highest toxicity. The aglycons, such as solanidine, have a more than 100-fold lower toxicity. α-Solanine and α-chaconine make up more than 95% of the total glycoalkaloid content in potatoes. Other glycoalkaloids are for example tomatine, tomatidenol and demissidine. In the context of the present disclosure, the level of glycoalkaloids is expressed as the sum of all glycoalkaloids. In case of potatoes this predominantly consists of α-solanine and α-chaconine.
Glycoalkaloids have a bitter taste and negatively affect many of the physical and/or biological properties of potato proteins, especially when the pH is increased by adhering to the soluble proteins as shown in the present disclosure. For food applications, the taste threshold of glycoalkaloids is about 140-170 mg of glycoalkaloids expressed as α-solanine per kg of product. This threshold strongly limits the applications of known native potato protein isolates in foods.
Various attempts have been made to remove glycoalkaloids. WO-97/42834, for instance, discloses a partial removal of glycoalkaloids by various ultrafiltration methods at excessive diafiltration conditions. Ultrafiltration can remove some glycoalkaloids and salts, but does not remove contaminants of high molecular weight, such as pectines, polyphenols and proanthocyanidines and colored derivatives thereof, such as epicatechins and anthocyanines, that are formed at pH values below 4.5.
Houben et al., J. Chromatogr. A, 1994, 661, 169-174 have employed a HPLC method which, however, does not detect the aglycons that are formed by enzymatic hydrolysis after prolonged processing of potato juice.
In DE 100 60 512 it has been proposed to remove glycoalkaloids from potato proteins by acidic extraction. This method, however, is not suitable for achieving glycoalkaloid levels below 100 ppm. Furthermore, this method can only be employed for precipitated or coagulated protein, and not for native, soluble protein.
Another method for removal of glycoalkaloids that has been suggested is enzymatic hydrolysis. This method, however, does not lead to removal of aglycon, which also binds to the potato proteins with negative effects on their physical and biological properties.
Fermentation is deemed unsuitable for safe removal of glycoalkaloids in the production of native potato proteins. Conversion by fermentation causes severe technical problems when implemented at commercial scale. The bioconversions are costly and have a low productivity. The micro-organisms that are used and their metabolites may end up in the protein product, which is undesirable.
One of the major problems in the isolation of potato proteins is caused by the common method of recovering the potato protein from the effluent of potato starch mills, which involves heat coagulation. Attempts to isolate the proteins from the potato juice using milder methods, such as membrane filtration and precipitation by heat or acid treatment, have proven to be inefficient on industrial scale. Membrane filtration applied directly to unclarified and clarified potato juice has proven to be very complicated and inefficient due to heavy fouling of the membranes and concomitant loss of flux and separation ability. Both membrane filtration and precipitation methods have significant drawbacks when applied directly to the potato juice due to the lack of selectivity between the desired protein product and other components in the raw material. Membrane filtration, for example, cannot separate the high molecular weight protein product from polymerized phenolic compounds or polysaccharides since the membrane will tend to retain them all. These compounds form complexes with potato proteins and result in a poorly soluble protein and low functionality in applications.
In the European patent application no. 06077000.5, an improved method for isolating native proteins from potatoes has been disclosed. This method comprises subjecting potato fruit juice to a flocculation by a divalent metal cation at a pH of 7-9, centrifuging the flocculated potato fruit juice, thereby forming a supernatant, subjecting the supernatant to expanded bed adsorption chromatography operated at a pH of less than 11 and a temperature of 5-35° C. using an adsorbent capable of binding potato protein, thereby adsorbing the native potato protein to the adsorbent, and eluting at least one native potato protein isolate from the adsorbent with an eluent. This method constitutes a significant improvement over earlier attempts to isolate potato proteins in that the potato proteins are obtained in native, i.e. non-denatured, form and in that a very high purity may be reached.
Nevertheless, it has been found that the method may not always reach sufficient removal of glycoalkaloids, particularly when variations in raw materials are encountered. Depending on the potato variety, the level of glycoalkaloids in the fruit juice may vary considerably. Variations of a factor 4-10, or more, are common in starch potato processing. For instance, the cultivars Seresta and Kuras contain more than 110-200 ppm some cultivars up to 300 ppm glycoalkaloids in fresh weight potato, whereas an Aveka cultivar contains only 30 ppm in fresh weight potato. The glycoalkaloids tend to adhere to or co-fractionate with the proteins. Potatoes that contain 1-1.5% soluble protein will lead to protein solutions than contain more than 300 to 4000 ppm glycoalkaloids on protein basis.
Also, glycoalkaloid levels may vary per variety depending on the harvesting season and weather conditions. It has been found that the method disclosed in the European patent application no. 06077000.5 may be difficult to adjust to cope with the variations in glycoalkaloid level, particularly when these variations are higher than 200 ppm. As a result, it may happen that the potato protein isolates obtained contain unsatisfactory amounts of glycoalkaloids.
There is thus still a need for a simple and effective method to remove glycoalkaloids from process streams encountered during isolation of potato proteins in native, soluble form on an industrial scale.